This invention relates to absorbent compositions of matter, specifically absorbent polymeric compositions of matter which are capable of absorbing large amounts of aqueous fluids. In particular, the invention relates to methods for chemically altering absorbent polymeric compositions to increase their ability to retain absorbed fluids even when subjected to physical stress, and the modified polymeric products produced by such methods. Final articles of manufacture having enhanced absorbency characteristics are provided.
The employment of various articles of manufacture to absorb a wide range of fluids in instances where fluids are encountered are well known in the art. Such known articles include diapers for infants and toddlers, incontinence garments, sanitary napkins, wound dressings, and emergency spill clean-up pads, to name but a few. Such articles typically comprise the absorbent material, and a suitable carrier structure ancillary to the absorbent useful for positioning and maintaining the absorbent in a location most conducive to the maximized efficient absorption of the liquid substance to be controlled. Often, the carrier structure comprises multi-layer film composites of varied configurations and having adhesive portions which assists in the affixation of the article as a whole to a portion of a person""s body, or other desired physical location.
Many polymeric and non-polymeric materials are known in the art as being useful as adsorbents for aqueous fluids. Perhaps the most notable of the suitable materials are those often referred to as xe2x80x9csuper-absorbing polymersxe2x80x9d or xe2x80x9cSAPsxe2x80x9d by those skilled in the art. Superabsorbents which are commercially available are cross-linked polyacrylic acids or cross-linked starch-acrylic-acid-graft-copolymers, the carboxyl groups of which are partially neutralized with a hydroxide of sodium or potassium. U.S. Pat. No. 5,409,771 of Dahmen et al. (the entire contents of which are herein incorporated by reference thereto) describes such superabsorbents. Typically, these materials can absorb many times their its weight in water and other aqueous solutions. The most preferred method of preparation of these materials involves the free-radical copolymerization of acrylic acid with a multifunctional crosslinking agent, such as trimethylolpropane triacrylate. Once produced, the polymer gel is then partially ionized by addition of alkali hydroxide, dried, and ground into a fine powder. In such form, the material may absorb 40 to 1000 times its weight in aqueous liquids.
One negative effect observed in connection with the use of such absorbing polymers is that retention of absorbed fluid is severely reduced on exposure of the moisture-swollen gel to an externally-applied weight or other force, such as when a urine-laden diaper is subjected to the forces encountered by the sitting action of a child to whom such diaper is affixed, to the extent that aqueous fluid may be caused to exude from the superabsorbent. Such exudation represents a significant drawback in the performance of such materials in sanitary articles, and there thus exists a need for further modification of these materials towards improving fluid retention when placed under stress.
One method known in the art for further modifying cross-linked acrylic acid copolymers involves esterification of the acrylic acid repeating units by reaction with diols, thereby further crosslinking the material according to the scheme: 
A diol which is commonly preferred for such esterification is 1,4-butanediol. However, when using 1,4-butanediol for such reactions, the reaction temperature required (c.a. 190xc2x0 C.) results in partial degradation of the polymer. Additionally, unacceptably high amounts of free unreacted diol remain after the reaction has been performed.
One desirable substitute for 1,4-butanediol in the esterification of cross-linked acrylic acid copolymers per the above are cyclic alkylene carbonates. The prior art U.S. Pat. No. 5,409,771 describes the use of alkylene carbonates in just such an employment. However, according to the invention claimed therein, temperatures of at least 150 degrees centigrade (and more preferably 180xc2x0 C. to 250xc2x0 C.) are required to effectuate the desired reaction. Unfortunately, partial degradation of the material desired to be produced is observed when using temperatures within these ranges. Thus, if there were a method for esterifying cross-linked acrylic acid copolymer superabsorbents without degrading the desired product, such a method would beneficially provide increased product yields and a product of overall higher quality than those currently available. Further, if such esterification could be conducted at lower temperatures using environmentally-friendly, biodegradable materials, such would be an advance in this art.
The present invention provides a process for providing an esterified copolymer useful as a super absorbent comprising the steps of 1) providing a copolymer having an esterifiable acid group; 2) providing (optionally) an alcohol; 3) providing water; 4) providing a esterification agent (an alkylene carbonate or polyol); 5) providing a catalyst; 6) mixing said copolymer, alcohol, optional water, esterification agent, and catalyst to form a mixture; and 7) heating said mixture to a temperature sufficient to cause esterification of said esterifiable acid group with said esterification agent.
Preferred catalysts useful in the invention are polyethers of the formula: 
having molecular weights of at least about 400 and wherein X is selected from the group consisting of: hydroxy and C1-C6 alkoxy, straight chain or branched, but is preferably methoxy when an alkoxy is selected; and R1, R2, R3, and R4 are each independently selected from the group consisting of: hydrogen, methyl, or ethyl; and n is any integer between 9 and 15,000.